Don't clear trailing whitespace

[levouh]

Describe the problem

Markdown defines that two trailing spaces means a new line, hence trailing whitespace shouldn't be cleared from the document when running mdformat (see here).

Link to your repository or website

No response

Steps to reproduce

1. Create a .md file
2. Write a line, say a block quote, that has trailing whitespace like:

>this line has two trailing spaces after it  
>so that this is actually a newline

3. Run mdformat on the file
4. Observe the failure to preserve the whitespace

The version of Python you're using

3.9

Your operating system

Linux 5.12 kernel

Versions of your packages

mdformat 0.7.7
mdformat-gfm 0.3.2
mdformat-tables 0.4.1

Additional context

No response

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[levouh]

Closing per the documentation stating:

Hard line breaks are always a backslash preceding a line ending. The alternative syntax, two or more spaces before a line ending, is not used because it is not visible.